1. Field of the Invention
The present invention relates to an electronic ballast with a full bridge circuit for controlling the operating behavior and brightness of a gas discharge lamp, and to a method for controlling the brightness of a gas discharge lamp.
2. Description of the Related Art
Electronic ballasts with full bridge circuits are preferentially employed for the operation of high pressure gas discharge lamps, but find employment also for low pressure discharge lamps or fluorescent tubes as well. Thereby, the use of a full bridge circuit offers the possibility to operate the lamps with a d.c. current, if applicable with low frequency polarity reversal, through which the arising of disruptive electronic magnetic alternating fields can be reduced. Further, in this case the influence of the lamp wiring on operation, arising as a result of the high frequency conductor impedances, is negligible. Ballasts with full bridge circuits are described for example in DE 44 01 630 A1 or AT 392 384 B.
The basic principle of a full bridge circuit is illustrated in FIG. 6 and will be briefly explained below. The full bridge circuit is constituted by means of four controllable switches S1 to S4, which in the present example are field effect transistors, the two first switches S1 and S2 forming a first half-bridge and the two switches S3 and S4 forming a second half-bridge. As the load of the full bridge circuit there is arranged in its diagonal branch a series resonance circuit consisting of an inductance L and a capacitor C, i.e. the series circuit of the inductance L and the capacitor C connects the common node point between the two switches S1 and S2 of the first half-bridge with the common node point between the two switches S3 and S4 of the second half-bridge. Gas discharge lamp LA is arranged parallel to the capacitor C. The input of the full bridge circuit is fed with a d.c. voltage UBUS, the output of the full bridge circuit is connected via a resistance R with ground.
The controlling of the four switches S1 to S4 is effected by means of two driver circuits T1 and T2 to which in turn corresponding control commands for the control of the switches S1 to S4 are passed from a regulation circuit 6. The control of the four switches S1 to S4 is effected as a rule in the following manner.
Initially, in a,first phase, the switches S1 and S4 forming a first bridge diagonal are activated, while the two switches S3 and S2 forming the second bridge diagonal are opened. In this first phase, a current flow takes place from the input of the full bridge circuit via the first switch, the load circuit consisting of the series resonance circuit and the gas discharge lamp LA, and the switch S4. Thereby, one of the two switches, for example the switch S1 is permanently closed, while the switch S4 is clocked at high frequency. With the switching frequency of the switch S4 remaining the same, the power delivered to the lamp LA is increased or reduced through alteration of the duty ratio. In a second phase, the switches S1 and S4 of the first bridge diagonal are then opened whereas now in analogous manner the switches S3 and S2 of the second bridge diagonal are activated, i.e. the switch S3 is permanently closed, whereas the switch S2 is clocked at high frequency with a duty ratio corresponding to the desired power. The change-over between the two bridge diagonals has the consequence that the direction of the current through lamp LA is permanently changed, through which mercury deposits on an electrode are avoided and the lifetime of the lamp is increased. The control of the full bridge circuit is assumed by means of the control circuit 6 to which on the one hand a desired value ISOLL corresponding to the desired lamp brightness is fed and on the other hand the voltage dropped via the shunt resistance R is fed via the input line 7 as actual value. In correspondence with the result of comparison between actual value and desired value, the control circuit 6 generates control commands which are delivered via the lines 81 to 84 to the two driver circuits T1 and T2, which in turn translate the control commands into corresponding signals for the control of the gates of the four field effect transistors S1 to S4.
The clocked switch of the bridge diagonal active in each case is opened and closed with a frequency of ca. 20 to 50 kHz. Due to this high frequency clocking, parasitic currents flow via the lamp line capacitances, which make an exact regulation of the lamp brightness, in particular at very low dimming values, impossible, with the consequence that at very low dimming values an undesired flickering of the lamp brightness, perceptible for the eye, appears.